Low-if pre-preamble antenna diversity receiver

ABSTRACT

An antenna diversity receiver has a low-IF receiving mode and a diversity selection mode, and a switch arrangement for coupling one of the antennas to the IF channels when in the receiving mode, and coupling each of the channels to a different one of the antennas in the selection mode. A diversity controller compares qualities of signals received simultaneously from the different antennas during the selection mode, and controls the switch arrangement in the receiving mode, to use the better antenna. The quality measurement is carried out during reception of a carrier before a preamble to a desired signal is received, so that there is more time to obtain a better signal quality measurement and to enable better averaging over time. The channels have a polyphase filter with switchable cross coupling between the channels, so that it acts as two independent low pass filters in the selection mode.

FIELD OF THE INVENTION

The invention relates to antenna diversity receivers, to terminalshaving such receivers and to methods of operating communicationsservices over such terminals.

BACKGROUND

Radio communication systems often suffer from the effects of multipathpropagation, whereby a transmitted signal reaches a receiver via aplurality of distinct paths from the transmitter. One solution to thisproblem is antenna diversity, in which two or more receiving antennasare provided for a receiver. Provided the antennas are sufficientlyseparated so that the signals received at one antenna are substantiallyuncorrelated with those received by another, when one antenna is in anull another antenna is likely to be able to receive a good signal. Anexample of a radio communication system which may make use of antennadiversity is a Bluetooth network, operating according to thespecification defined by the Bluetooth Special Interest Group. Such anetwork is intended to provide low-cost, short range radio links betweenmobile PCs, mobile phones and other devices, whether portable or not.Communication in a Bluetooth network takes place in the unlicensed ISMband at around 2.45 GHz. At such frequencies, antenna separations of theorder of a few cm are sufficient for successful diversity operation.

In an antenna diversity receiver, for example that disclosed in U.S.Pat. No. 5,940,452, a diversity controller selects the antenna providingthe best signal according to a signal quality measurement, which is mostcommonly the RSSI (Received Signal Strength Indication). Other measuresof channel quality can be used, for example checksums are used incertain cases in a DECT (Digital Enhanced Cordless Telecommunications)system. In a radio communication system in which data is transmitted inpackets, it is preferable for the diversity controller to select theoptimum antenna on a packet-by-packet basis. This is particularly thecase in a frequency-hopping system such as Bluetooth, because successivepackets will be sent on different frequencies whose characteristics willnot be correlated.

However, implementation of antenna diversity on a packet-by-packet basisrequires measurement of the signal quality from each antenna in turn(unless a plurality of receivers is provided, which is not generally apractical solution). The sequential RSSI measurement process employed inknown receivers may therefore take too long, particularly if thepreamble to each packet is short (for example, that in Bluetooth is only4 μs long). Accordingly, it was proposed in patent application WO0203570 to provide an antenna diversity receiver enabling simultaneouscomparison of signal quality from two antennas without the need for aplurality of receivers. This involved modifying the switch which selectseither the signal from the first antenna or the second antenna, andcouples that signal to both in-phase and quadrature channels. Themodified switch is able to route the signal from the first antenna toone of the channels and route the signal from the second antenna to theother of the channels. The resultant signals are folded around zerofrequency, and therefore cannot be demodulated, but valid signalstrength measurements may still be taken for each antenna. A diversitycontroller is able to compare the signal qualities from the two antennasduring a preamble in the transmitted data and to determine which antennato use for the remainder of the data.

The same patent application indicates that in low-IF (IntermediateFrequency) architectures employing a polyphase filter rather than lowpass filters, additional circuitry would be needed for deriving a signalfrom each of the I and Q channels before they pass through the polyphasefilter. Each of these derived signals would then need to be passedthrough a separate channel filter, to filter out adjacent channelsignals, before signal quality measurements could be made. This involvesmore cost and complexity. In any case, the selection of which antenna touse is still poor because the preamble is so short that the selection isheavily susceptible to random noise during the preamble.

SUMMARY OF THE INVENTION

It is an object of the invention to provide improved apparatus ormethods, addressing such problems. According to a first aspect of theinvention, there is provided an antenna diversity receiver for use withtwo or more antennas and being arranged to operate in a low-IF receivingmode and in a diversity selection mode, the receiver having: IFchannels, a switch arrangement for coupling one of the antennas to thechannels when in the receiving mode, a diversity controller forcomparing qualities of signals received simultaneously from thedifferent antennas during the diversity selection mode, and controllingthe selection by the switch arrangement in the receiving mode, dependingon the comparison, the receiver being arranged to operate in theselection mode during reception of a carrier before a preamble to adesired signal is received.

By using the carrier which may be unmodulated, for the measurements,rather than only a preamble, notably there is likely to be more time toobtain a better signal quality measurement. More time can enable betteraveraging over time and hence reduced susceptibility to noise. More timecan also enable slower or lower tolerance components to be used, toenable lower costs.

A zero-IF (Intermediate Frequency) receiver cannot achieve this in thesame way because the unmodulated carrier will produce a DC receivedsignal, which cannot be adequately distinguished from the unwanted DCoffsets that will be present at the output of the two channel filters.

Notably, signals from both or all antennas can be measured at the sametime, which has advantages over conventional schemes for measuring oneantenna then another sequentially. This means the overall time taken canbe reduced, typically by half. A consequence of this is that the powerconsumption can be reduced, which is particularly important for smallbattery powered receivers. Also, by measuring simultaneously, themeasurement time for each signal as a proportion of the overall time isincreased, so the measurement can be averaged over a longer period,which can give greater immunity from noise and greater accuracy if thesignal strength is varying with time. Also, by measuring simultaneously,there is no inaccuracy caused by the signal strengths into both antennasvarying during the measurement period.

Notably it is not essential that the measurement has to be completedbefore the preamble. If it has not been possible to decide which antennahas the strongest signal before the preamble, then measuring cancontinue during the preamble. Of course this means reception of thepreamble (or any part of the signal) cannot be confirmed until thereceiver is switched to receiving mode.

An additional feature of some embodiments is the channels having acomplex filter with cross coupling between the channels, for outputtingasymmetrically filtered channel signals in the receiving mode, thecontroller being arranged to use independent low-pass filtered channelsignals without coupling between the channels, for the comparison in theselection mode.

Although it is possible in principle to use other filter arrangements,or other signals for the comparison, they would be less convenient andso more expensive to implement.

An additional feature of some embodiments is the filter being apolyphase filter in the form of a complex filter with an asymmetricalfrequency response centred around a low-IF with controllable crosscoupling between the channels, the controller being arranged to useoutputs of the polyphase filter with cross coupling suppressed duringthe selection mode.

This is particularly useful as it enables the amount of additionalcircuitry to be kept low, thus keeping costs and size low. This can alsoresult in reduced power consumption in the polyphase filter if thecross-coupling is powered down in the selection mode, As the uncoupledparts of the polyphase filter have a low pass characteristic, there isno longer a need to provide separate low pass filtering for the channelsignals used for the comparison.

An additional feature of some embodiments is the diversity controllerbeing arranged to measure the signal quality of the signal received fromthe selected antenna in the receiving mode. This can be used tocorroborate measurements made on the unmodulated carrier in theselection mode.

An additional feature of some embodiments is the diversity controllerbeing arranged to cause the switching arrangement to couple another ofthe antenna to the channels if the signal quality in the receiving modeis below a threshold. This can be helpful in the case that themeasurements made on the received signal are subject to moreinterference than the measurements on the carrier.

An additional feature of some embodiments is a pass band of thepolyphase filter being broadened during the selection mode. This isuseful to ensure that measurement accuracy is not reduced by parts ofthe signal being suppressed by the filter before they are measured. Thismight happen in an extreme case such as the carrier before the preamblebeing modulated with a constant “1” or if the intermediate frequency issignificantly higher than the bandwidth of the signal.

An additional feature of some embodiments is the diversity controllerbeing arranged to control switching between the selection Mode and thereceiving mode according to predetermined time intervals. This isusually arranged to conform to standards according to the type ofreceiver. In principle an alternative or addition is to determinetimings from the received signals.

An additional feature of some embodiments is the receiver beingcompatible with the Bluetooth standard. This is one of the moreimportant commercially valuable standards.

An additional feature of some embodiments is the controller beingarranged to determine a received signal strength indication for eachsignal. This is a suitable, commonly used measure.

An additional feature of some embodiments is amplifiers for amplifyingsignals from each of the antennas, arranged so that in the receivingmode, amplifiers corresponding to unused ones of the antennas can beswitched off. This can enable power consumption to be kept low, which isparticularly important for small mobile battery powered terminals.

An additional feature of some embodiments is having a local oscillatorfor quadrature mixing with the signals from the antenna, the localoscillator being arranged to have a different frequency for theselection mode, to enable zero-IF operation. This is an alternative todisconnecting the cross coupling of the filter, but has disadvantages ofbeing slower and needing more circuitry, so can be more expensive.Another alternative is to use a lower local oscillator frequency duringselection mode in conjunction with disconnecting cross-coupling, suchthat the signals from the two antennas pass optimally within thebandwidth of the two low-pass filters when in selection mode. During thereceive mode a higher intermediate frequency can be used with the higherlocal oscillator frequency.

An additional feature of some embodiments is the controller beingarranged to use the channel signals before the filter, and apply lowpass filtering to each of them to determine the signal qualities. Thisis another alternative to disconnecting cross coupling, but has thedisadvantage of needing more circuitry.

An additional feature of some embodiments is the receiver beingimplemented as one or more integrated circuits.

Another aspect of the invention provides a mobile terminal having anantenna, the receiver of any preceding claim coupled to the antenna, anda processor coupled to the receiver for processing data received by thereceiver. This is claimed explicitly as the advantages can feed throughto enable better terminals, with much greater added value than the valueof the receiver component alone.

Another aspect of the invention provides a method of offering acommunication service using the terminal. This is claimed explicitlyagain as the advantages can feed through to provide improved serviceswith much greater added value than the value of the receiver componentalone.

Any of the additional features can be combined together or with any ofthe aspects of the invention, as would be apparent to those skilled inthe art. Other advantages may be apparent to those skilled in the art,especially over other prior art not known to the inventors.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of example,and with reference to the accompanying drawings, in which:

FIG. 1 shows an embodiment of a receiver for use with a wireless linkand a network,

FIG. 2 shows operations of a receiver,

FIG. 3 shows an embodiment of a receiver, when in a selection mode,

FIG. 4 shows a graph of a frequency response of a switchable polyphasefilter in the selection mode,

FIG. 5 shows an embodiment of a receiver when in receiving mode,

FIG. 6 shows a graph of a frequency response of a polyphase filter whenin the receiving mode, and

FIG. 7 shows an example of a poly phase filter.

DETAILED DESCRIPTION

FIG. 1, Embodiment of a Receiver Coupled to Network.

FIG. 1 shows an embodiment of a receiver 60 coupled by a wireless linkto another wireless terminal 3 which can be incorporated in any type ofdevice. The link can be a bluetooth link or other types. The otherterminal can be in a stand alone device or can be coupled to a networkor networks 90. One particular application is for receiving data relatedto communications services offered by an operator over the network, suchas voice services or information services for example. The receiver inthis example is incorporated in a mobile terminal 70, though inprinciple it could be applied in a base station. The mobile terminalincludes a pair of antennas 1, 2, suitable for spatially diversereception. Other parts of the mobile terminal such as user interface,data or voice processing functions and so on are represented by part 80.The receiver can be applied in any type of terminal. Typically thereceiver will be incorporated in a transceiver, though transmitter partsare not shown for the sake of clarity.

The receiver includes a switch arrangement 10, I and Q channels 20, 30,and a diversity controller 50. Optionally there are other parts of thereceiver not illustrated, such as baseband processing of the channels. Afeature of a receiver is that two channels (normally I and Q) arerequired for normal receiving, but each channel can be fed with adifferent signal during a period of signal quality measurement andcomparison. The switch couples the antennas so that one antenna iscoupled to both channels in a receiving mode. In a selection mode, thesignals from each antenna are processed separately so that therespective signal qualities can be measured simultaneously and compared.The antenna with the better quality is used in the receive mode. Thediversity controller enters the selection mode often enough to updatethe selection as reception conditions change, particularly if the mobileterminal is moving. It can be repeated before each packet or before asequence of packets for example.

The same I and Q channels as used in the receiving mode can be switchedand used for processing the signals to obtain signal qualitymeasurements, as illustrated. Alternatively, in principle, duplicatechannels can be used, though this would add circuitry and costs. Thereceiver can be implemented as an integrated circuit.

FIG. 2, Receiver Operation

FIG. 2 shows steps in the operation of the receiver of FIG. 1. In a lefthand column the state of the signal at the antennas is shown. In thecentral column the state of the switching arrangement is shown. In theright hand column the operation of the diversity controller isindicated. In the first row, the signals received are unmodulatedcarrier signals or other unwanted signals or in between desired signals.The diversity controller operates to control the switch arrangement tofeed different antenna signals to different channels. The controllermeasures the signals and compares the signal qualities. This measurementcan be carried out over a period of time so that a decision is based onaverage qualities, to reach a decision on which antenna to use forreceiving the subsequent desired signal. The desired signal may bepreceded by a preamble as shown in the second row of FIG. 2. At any timebefore or during the preamble, after the signal quality measurement hasbeen made, the controller changes the mode from section to receivingmode. The preamble can be defined as a sequence of received signalprepending the information part of the signal for purposes of timingsynchronisation and/or wanted signal recognition in the receiver.

The switch arrangement as shown in the second row changes so that oneantenna, the one selected as a result of the preceding selection mode,is coupled to both channels. The preamble, depending on the airinterface standard, may provide enough time to enable the actual signalstrength of a demodulated signal to be measured. This can be measured interms of RSSI as defined in the Bluetooth or other wirelessspecifications, or other arbitrary measures of signal strength could beused to drive the diversity controller or be converted to the requiredRSSI format if desired. Optionally this can be carried out by thecontroller and compared to a threshold. If too low, the controller cantry switching to the other antenna before the desired signal arrives.The third row shows the desired signal in the form of a packet of datarelating to a communications service arriving at the antennas. Thediversity controller is in receive mode in time for the receiver toreceive every bit of the packet that is to be received. The switcharrangement is set to couple the selected antenna to both channels.After the packet or string of packets, the controller can return to theselection mode.

FIGS. 3, 4 Embodiment of a Receiver, in Selection Mode

A block schematic diagram of an embodiment of a receiver is shown inFIG. 3 in the selection mode. The same embodiment is shown in FIG. 5using corresponding reference numerals, when in the receiving mode. Thereceiver can be an example of the receiver of FIG. 1 or other receivers.Radio Frequency (RF) signals are received by first and second antennas100, 110 passed through antenna filters 120, 130, and amplified byrespective Low Noise Amplifiers (LNA) 170, 160. At this stage the RFsignals are contain a first frequency band which contains a wantedsignal, while second and third frequency bands contain unwanted adjacentchannel signals.

The outputs of the LNAs are connected to quadrature-related mixers 220,210 via a switching arrangement in the form of first and second two-wayswitches 180, 190 controlled by a diversity controller (DC) 230 whichcompares the received signal qualities. The DC is also coupled to theLNAs to switch off the power to whichever LNA is unused in the receivingmode. When both switches are in the ‘up’ position, as shown in FIG. 5discussed below, the receiver behaves in the same way as a conventionallow-IF receiver, receiving and processing signals from the firstantenna. Similarly, when both switches are in the ‘down’ position,signals are received from the second antenna only. Although two antennaare illustrated, it is quite possible to have more than two, with multiway switches and a more complex DC to carry out multiple comparisons forexample.

The pair of quadrature-related mixers are supplied with In-phase (I) andQuadrature (0) Local Oscillator (LO) signals respectively. The LOsignals are generated by a Voltage Controlled Oscillator (VCO) 200,driven by a frequency synthesiser (SYN) 150 having a stable referencesignal source 140. The LO signals are offset from the centre of thefrequency band including the wanted signal, so the mixers mix the signalto an intermediate frequency. The signals now have a frequency bandcontaining the wanted signal with other frequency bands above and belowcontaining the adjacent channel signals.

The output signals from the I and Q mixers are then amplified by IFamplifiers 250, 240, then filtered by polyphase I and Q channel filter260. This filter acts as a channel filter in the receiving mode and as apair of low pass filters in the selection mode. Polyphase filters arecomplex active filters with an asymmetrical frequency response centredaround a low Intermediate Frequency as shown in FIG. 6. They are usefulbecause they can be used in a low-IF receiver in which unwanted DCoffsets are blocked at the output of the demodulator simply by puttingcapacitors to ac-couple the received signals. This is usually easier toimplement than DC-resetting circuitry (or better than tolerating the DCoffsets) in a typical zero-IF receiver. Polyphase filters are typicallyimplemented either with op-amps and resistors or with gyrators andcapacitors as shown in FIG. 7. In both cases a polyphase filter iscomposed of a real low-pass filter in each of the I and Q paths and alsocomponents for cross coupling I to Q and Q to I. This doubles the sizeof the two real low-pass filters alone. It is the cross-coupling thatshifts the frequency response to the desired IF. If the cross-couplingis disconnected or powered down, as shown in FIG. 4, the centrefrequency of the pass band shifts to zero-IF and the bandwidth stays thesame unless altered as described below.

Simultaneous signal quality measurements (or comparisons) can be madewhen the first switch 180 is in the ‘up’ position and the second switch190 is in the ‘down’ position. Signals from the first antenna are fed tothe I mixer 220, mixed down to the IF frequency and filtered. Theresultant signal cannot be demodulated, but a valid signal strengthmeasurement may still be taken. Similarly, signals from the secondantenna are fed to the Q mixer 210 and can have their strength measuredafter filtering. The fact that signals from the first antenna are mixedwith an in-phase LO signal and those from the second antenna are mixedwith a quadrature LO signal is of no consequence as it has no effect onthe measured signal strength.

The diversity controller is able to compare the signal qualities fromthe two antennas before a preamble in the transmitted data and todetermine which antenna to use for the remainder of the data. The timingof the change of mode from selection mode to receiving mode can be setaccording to predetermined intervals according to the air interfacestandard. In the case of Bluetooth, the receiver knows within certaintolerances when to expect a packet, and the duration of a preamblebefore the packet. These timings are monitored by the DC. If thecomparison is not completed in time, the DC can opt to continue theselection mode into the preamble.

Generally the received signal before the preamble is typically muchlonger than the preamble, perhaps even longer than the packet, at leastfor Bluetooth signals. This is because it takes a long time to lock thetransmit local oscillator to the required frequency within allowedtolerances and because often powering up a power amplifier in thereceiver will pull the Local Oscillator LO off frequency, so the LOneeds to be (fine-)tuned while the PA is on and consequently during thistime it is transmitting something and the receiver can see the signal.

FIGS. 5, 6 Embodiment of a Receiver, in Receiving Mode

FIG. 5 shows the same embodiment as that of FIG. 3, when in thereceiving mode. When the comparison of signals from the antennas iscomplete, and a decision has been taken, the DC enters the receivingmode, as shown in FIG. 5, sets the switches accordingly, and controlsthe polyphase filter so that the receiver functions as a normal low-IFreceiver. The DC also controls the LNAs so that the LNA connected to theunused antenna is switched off during data reception, thereby minimisingreceiver power consumption. The functions of the DC can be implementedin conventional circuitry with analog and digital parts, for example onthe same integrated circuit as the other parts of the receiver.

In the receiving mode, the output signals from the polyphase filter arecoupled by capacitors 290, 280 to I and Q limiters 300, 310 before beingconverted to digital signals by I and Q single-bit Analogue-to-DigitalConverters (ADC) 330, 320. The limiters remove amplitude informationprior to the inputs to the ADCs. The digital signals are then passed toa Baseband processing block (BB) 340 where they are demodulated. Theeffects of the spectrum folding can be removed by the basebandprocessing by use of the I and Q channels.

Compared with a conventional low-IF receiver, the receiver of theembodiment shown requires only a small amount of extra circuitry (oneLNA and two switches in the illustrated embodiment) and makes only asmall increase in the receiver's power consumption, especially if theextra LNA is switched off during data reception as suggested above.

Alternative structures for the receiver are conceivable, for example,the signal could be demodulated then passed to the base band section.Parts described such as the limiters, ac-coupling and ADCs for example,are all optional depending on the desired implementation.

FIG. 7, Polyphase Filter

FIG. 7 shows an example of a polyphase filter for use in the embodimentsdescribed above or in other embodiments. It is formed from two low passfilter arrangements using gyrators and capacitors, with cross couplingbetween the arrangements. Gyrators are well known for simulatinginductance and are made up of transistors and capacitors, which can bereadily formed in integrated circuits, unlike inductors. In thisillustration, each low pass filter arrangement is formed from a chain ofgyrators 400 with a parallel coupled capacitor 450 across the input ofeach gyrator. The cross coupling is provided by links coupling theinputs of each gyrator with its counterpart in the chain if the otherarrangement. Another gyrator 420 is provided in each of the crosscoupling links. The inputs of each chain show a signal source 430 and aninput impedance 440. The length of the chain can be chosen according tothe desired filter characteristics following established principles. Tomake the polyphase filter switchable, a mechanism is provided forsuppressing the cross coupling. This can be achieved simply by reducingor removing the power supply to the gyrators 420 in the cross couplinglinks, or by other means such as switches in the links.

The frequency response of the filter is shown in FIGS. 4 and 6 asdescribed above. One further enhancement is to alter the response tobroaden the pass band for the selection mode. This is useful to addressthe issue of an extreme case such as the signal before the preamblebeing modulated with constant “1”. This could take it out of band of thenow low-pass filter and consequently it could be reduced in strength,and so the measurement would be less accurate. As the same happens forboth antennas, the comparison will still be valid. Also, the measurableRSSI is well below the signal level that can be usefully demodulated(i.e. below Sensitivity signal strength), so even if reduced by say 16dB, it is still enough to be measured. Nevertheless, by widening thefilter bandwidth as well as making it low-pass in the selection mode,this issue can be avoided. Typically, provision for varying thebandwidth is built-in anyway to allow it to be tuned optimally forprocess spreads. This can be done by switching components of the filterin or out, following established design principles.

Other Variations and Concluding Remarks

Variations on the receiver design are possible. For example, thelimiters could be removed and the single-bit ADCs substituted bymulti-bit ADCs, enabling signal strength measurements to be takendirectly from the digital part of the receiver. The embodiment disclosedabove is a direct-conversion receiver, in which the RF signal is mixeddown directly to the intermediate frequency. However, the presentinvention is applicable to other low-IF architectures such as thoseemploying a two-stage down-conversion. The channels of the claims can beembodied by all or just parts of the chain of elements shown.

Embodiments have been described with reference to the Bluetoothspecification, though it will be apparent that it is applicable to othercommunication systems in which antenna diversity can be employed, forexample UMTS (Universal Mobile Telecommunication System), GSM (GlobalSystem for Mobile communications) or DECT.

One possible practical drawback with the polyphase filter described isthat when switched to operate as two low-pass filters it no longerrejects an interfering signal in the negative adjacent channel. If thereis a negative adjacent signal present then the antenna selection will bebased on the combination of the wanted and interfering signals. If theinterfering signal is much stronger than the wanted then the bestantenna for the wanted signal will be chosen by assessing the RSSI ofthe interferer and could therefore be wrong. Generally the interfererwill be modulated and the wanted won't be, so the interferer will bereduced in strength by the filter more than the wanted. There's a lowprobability of the interferer causing a problem in Bluetooth receiversgiven that there are 79 Bluetooth channels, and the worst that happensis that a less optimal antenna is occasionally chosen. For Bluetooth theexpected time of arrival can be in error by +−10 μs, so the systemshould be ready (in receiving mode, in other words, single antenna IQmode) at least 10 μs early. If the wrong antenna was chosen because of astrong negative adjacent interferer then when receiving the wantedsignal properly the measured RSSI will drop. If the RSSI drops below theSensitivity level, the signal cannot be received. If the Bluetoothsignal does not arrive early then there is probably still time tomeasure the RSSI and if it is too low, switch to the other antenna inthe hope that it will be better. This enhancement can help mitigate thisinterference problem.

A further option is a ‘passive polyphase filter’. This is another typeof polyphase filter that uses only passive components i.e. uses nocurrent. It has no low-pass frequency response. Instead it just rejectsnegative frequencies and passes all positive frequencies. So, if thesignals are passed though a passive polyphase filter before or after theactive polyphase filter and without doing anything different inselection or receive mode, then it will have no affect in receive mode(except perhaps helping to reduce the image resulting from poor IQimbalance) but in selection mode it will reject the negative adjacentchannel and thus remove the problem of making the antenna selectionbased on a strong negative adjacent interferer. It requires morecircuitry to implement this, but at least it requires no extra current.

As has been described above, an antenna diversity receiver has a low-IFreceiving mode and a diversity selection mode, and a switch arrangementfor coupling one of the antennas to the IF channels when in thereceiving mode, and coupling each of the channels to a different one ofthe antennas in the selection mode. A diversity controller comparesqualities of signals received simultaneously from the different antennasduring the selection mode, and controls the switch arrangement in thereceiving mode, to use the better antenna. The quality measurement iscarried out during reception of a carrier before a preamble to a desiredsignal is received, so that there is more time to obtain a better signalquality measurement and to enable better averaging over time. Thechannels have a polyphase filter with switchable cross coupling betweenthe channels, so that it acts as two independent low pass filters in theselection mode. Other variations and examples within the scope of theclaims will be apparent to those skilled in the art.

1. An antenna diversity receiver for use with two or more antennas andbeing arranged to operate in a low-IF receiving mode and in a diversityselection mode, the receiver having: IF channels, a switch arrangementto select one of the antennas to be coupled to the channels when in thereceiving mode, a diversity controller for comparing qualities ofsignals received simultaneously from the different antennas during thediversity selection mode, and controlling the selection by the switcharrangement in the receiving mode, depending on the comparison, thereceiver being arranged to make measurements of a carrier duringreception in the selection mode before a preamble to a desired signal isreceived.
 2. The receiver of claim 1, the diversity controller beingarranged to measure the signal quality of the signal received from theselected antenna in the receiving mode to corroborate the measurementsmade on the carrier in the selection mode.
 3. The receiver of claim 2,the diversity controller being arranged to cause the switchingarrangement to couple another of the antenna to the channels if thesignal quality in the receiving mode is below a threshold.
 4. Thereceiver of claim 1, the diversity controller being arranged to controlswitching between the selection mode and the receiving mode according topredetermined time intervals.
 5. The receiver of claim 1, beingcompatible with the Bluetooth standard.
 6. The receiver of claim 1, thediversity controller being arranged to determine a received signalstrength indication for each signal.
 7. The receiver of claim 1, furthercomprising amplifiers for amplifying signals from each of the antennas,the receiver being arranged so that in the receiving mode, amplifierscorresponding to unselected antennas can be switched off.
 8. Thereceiver of claim 1, further comprising a local oscillator forquadrature mixing with the signals from the antenna, the localoscillator being arranged to have a different frequency for theselection mode, to enable zero-IF operation.
 9. The receiver of claim 1,implemented as one or more integrated circuits.
 10. The receiver ofclaim 1, further comprising a processor coupled to the receiver forprocessing data received by the receiver.
 11. A method of communicationusing a mobile terminal having plural antennas, the method comprising:coupling each respective antenna of the plural antennas to a respectiveindependent low-pass filtered channel by a switch arrangement in adiversity selection mode; making measurements of a carrier receivedsimultaneously on the antennas in the diversity selection mode;comparing qualities of measurements taken from the multiple antennasduring the diversity selection mode; coupling an antenna of the pluralantennas to a plurality of channels by a switch arrangement in areceiving mode based on the comparing qualities of measurements; andcross-coupling the plurality of channels to output asymmetricallyfiltered channel signals in the receiving mode.
 12. The method of claim11, further comprising measuring a signal quality of a signal receivedfrom the selected antenna in the receiving mode to corroborate themeasurements made on the carrier in the selection mode.
 13. The methodof claim 11, further comprising switching between the selection mode andthe receiving mode according to predetermined time intervals.
 14. Themethod of claim 11 wherein the mobile terminal includes pluralamplifiers, each respective amplifier being coupled to a respectiveantenna of the plural antennas for amplifying signals therefrom, furthercomprising switching off at least one amplifier of the pluralamplifiers, wherein the antenna of the plural antennas that is coupledto the plurality of channels is not switched off in receive mode.
 15. Anantenna diversity receiver for use with two or more antennas arranged tooperate in a receiving mode and in a diversity selection mode, thereceiver comprising: IF channels comprising a complex filter withswitchable cross coupling between the channels, to output asymmetricallyfiltered channel signals in the receiving mode; a switch arrangement tocouple one of the antennas to the channels when in the receiving mode;and a diversity controller for comparing qualities of signals receivedsimultaneously from the different antennas during the diversityselection mode, and controlling the selection by the switch arrangementin the receiving mode depending on the comparison, the diversitycontroller configured to use independent low-pass filtered channelsignals without coupling between the channels for the comparison in thediversity selection mode.
 16. The receiver of claim 15, the complexfilter being a polyphase filter in the form of a complex filter with anasymmetrical frequency response centered around a low-IF withcontrollable cross coupling between the channels, the controller beingarranged to use outputs of the polyphase filter with cross couplingsuppressed during the selection mode.
 17. The receiver of claim 16wherein the polyphase filter comprises a broadened pass band during thediversity selection mode.
 18. The receiver of claim 15 wherein thecontroller uses the complex filter to apply low pass filtering to eachchannel signal to determine the signal qualities.
 19. The receiver ofclaim 15, the diversity controller being arranged to control switchingbetween the selection mode and the receiving mode according topredetermined time intervals.
 20. The receiver of claim 15, wherein thereceiver is configured to be compatible with the Bluetooth standard. 21.The receiver of claim 15, the diversity controller being arranged todetermine a received signal strength indication for each signal.
 22. Thereceiver of claim 15, further comprising amplifiers for amplifyingsignals from each of the antennas, the receiver arranged so that in thereceiving mode amplifiers corresponding to unselected antennas can beswitched off.
 23. The receiver of claim 15, further comprising a localoscillator for quadrature mixing with the signals from the antenna, thelocal oscillator being arranged to have a different frequency for theselection mode, to enable zero-IF operation.
 24. An antenna diversityreceiver for receiving a carrier, comprising: a plurality of antennasarranged to receive a carrier of a signal carrying data that is precededby a preamble; at least two channels, each channel configured todemodulate a respective component of the signal; a switch arrangementthat selectively couples a respective antenna of the plurality ofantennas to at least two channels when the receiver is in a receive modeand that selectively couples a first antenna of the plurality ofantennas to a first channel of the at least two channels and a secondantenna of the plurality of antennas to a second channel of the at leasttwo channels when the receiver is in a select mode; and a diversitycontroller coupled to the switch arrangement and the first and thesecond channels of the at least two channels and arranged to control theswitch arrangement in the receive mode and the select mode of thereceiver and compare a respective quality of the signal received at thefirst antenna during a time span before the preamble with a respectivequality the signal received at the second antenna during the same timespan before the preamble when the receiver is in the select mode. 25.The receiver of claim 24, comprising a complex filter switchablyarranged to cross couple the first channel and the second channels whenthe receiver is in the receive mode.
 26. The receiver of claim 25wherein the complex filter is switchably arranged to decouple the firstchannel and the second channels when the receiver is in the select mode.27. The receiver of claim 25 wherein the diversity controller isarranged to control the switch arrangement to select the respectiveantenna of the plurality of antennas that is coupled to the at least twochannels when the receiver is in receive mode depending on thecomparison of the respective qualities of the signal when the receiveris in select mode.
 28. The receiver of claim 24 wherein the diversitycontroller is arranged to determine a respective signal strengthindication of the signal at the first antenna and the second antenna,wherein the compared respective qualities of the signal are therespective signal strength indications.